1. Industrial Field of the Invention
The present invention relates to a method for forming diamond by high pressure synthesis employing an anvil press and using C.sub.60 and/or graphitic carbon microtubules as the starting powder, while heating said starting powder by laser irradiation. The present invention also relates to an apparatus for use in forming diamond by said high pressure synthesis.
2. Prior Art
Synthetic diamonds are now available by two types of processes. One of such process types is vapor phase synthesis as represented by chemical vapor deposition (CVD), and the other is high pressure synthesis which comprises bringing powders of graphite, diamond, etc., under high temperature and high pressure to obtain crystalline diamond therefrom. The high pressure synthesis are further classified into three methods as follows:
1. Impact process; the process comprises bringing the starting powder instantaneously under high pressure by applying impact which is generated by, for example, the explosion of explosives and the collision of a body accelerated to a high speed. In this manner, granular diamond can be obtained by directly converting the starting powder material having a graphite structure into a powder composed of grains having a diamond structure. This process is advantageous in that no press as is required in the two other processes is necessary; however, there is difficulty in controlling the size of the resulting diamond products.
2. Direct conversion process; the process comprises maintaining the starting powder under a high static pressure of from 13 to 16 GPa and a high temperature of from 3,000.degree. to 4,000.degree. C. in a sealed high pressure vessel. By thus realizing stability conditions for diamond, the powder material undergoes direct phase transition from graphite into diamond, through graphite decomposition and structural reorganization into diamond.
3. Flux process; the process is similar to the aforementioned direct process in that a static pressure and high temperature are applied to the starting material . In this process, however, fluxes such as Ni and Fe are utilized to allow the reaction to occur under lower pressure and temperature conditions as compared to those employed in direct conversion process. That is, in the present process, the entire atomic rearrangement which occurs in the conversion process is accelerated by the use of a flux. Specifically, this process comprises charging the flux inside a high pressure vessel in such a manner that it may be enclosed in the starting powder material, i.e., high purity graphite, and then, heating them wholly under a pressure of from 4 to 6 GPa in the temperature range of from about 1,500.degree. to 2,000.degree. C. Thus , the flux turns into a solution and graphite is then dissolved therein to give a saturated solution. Because the pressure inside the high pressure vessel at this point is maintained in the stability range for diamond, the solubility for graphite far exceeds that for diamond. Thus, what happens inside the vessel is diamond precipitation and dissolution of graphite into the flux.
Among the high pressure synthesis processes described above, the second and the third ones using a press are widely used because they enable single crystal diamonds as large as several millimeters in size. This was realized by the advent of apparatuses such as a belt-type apparatus and an anvil apparatus. In fact, such single crystals are unfeasible by vapor phase synthesis.
The aforementioned high pressure synthesis makes the best of a pressure member made from cemented carbide called an anvil. It is attempted at present to realize a further high pressure by using diamond anvils.
The high pressure syntheses described above suffer many problems yet to be solved, however. First comes the poor productivity as a common problem, ascribed to the use of a large scale pressure apparatus which sometimes is as large as several meters in size. There is proposed to increase the productivity by exchanging the high pressure vessels, etc., however, none of the steps can be omitted from the sequence comprising pressing, heating, cooling, and reducing the pressure, nor the steps can be proceeded simultaneously.
As described earlier, it is requisite in the high pressure synthesis to carry out continuously the steps of heating the pressed sample, cooling the sample after completion of the reaction, and taking the sample outside the vessel after reducing the pressure. It is also required that the sample is heated during applying pressure by means different from that for the application of the pressure. However, there was a problem with respect to the means for heating. That is, in a conventional high pressure synthesis, the sample was generally heated by directly applying an electric current thereto. However, the electric resistance of the sample increases with progress in the reaction. Thus, it can be seen that the sample temperature is hardly maintained constant. Accordingly, a process comprising heating by applying an electric current to the sample was unsuitable for synthesizing high quality single crystal diamond, which required maintaining the temperature constant for a relatively long time.
In a flux process employing heating by applying current, there was another problem of precipitating, on one hand, unreacted graphite at the vicinity of the electrodes and on the other, flux at the center between the electrodes. Those precipitates form such a manner as to enclose the diamond portions to give a heterogeneous sample as a result. This phenomenon occurs irrespective of the current applied, i.e., whether a DC or an AC is applied. This is certainly an unavoidable phenomenon, however, it is also true that this lowers the product yield.
As a measure to overcome the aforementioned problems, there is proposed to provide a heater around the sample. This technique, however, requires the sample volume to be considerably reduced as compared to that in heating the sample directly by applying an electric current. Accordingly, this method reversely contributed to the improvement of product yield.
There is also proposed to utilize a laser beam for heating. However, there is no method for using effectively the laser beam for heating the entire sample.
Though there are various problems as enumerated above, the process using a high pressure press still is a unique process which enables synthesis of single crystal diamond or synthetic diamond materials ranging in size from several micrometers to several millimeters. It is therefore important to modify the process so that the characteristics thereof may be fully exhibited.